Given a 1-D float array of monthly revenue in chronological order, return an array of the same length:

• element 0 is np.nan
• element i is (x[i] - x[i-1]) / x[i-1]
• a zero prior month yields np.nan

Input

[100. 110.  99.   0.  50.]

Output

[ nan  0.1 -0.1 -1.   nan]